CHEM2213 Topic 1: Acid-Base Reactions Notes.docx

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School

Western University

Department

Chemistry

Course

Chemistry 1027A/B

Professor

Mark Workentin

Semester

Fall

Description

Topic 1
Acid–Bases Reactions
What Are Arrhenius Acids & Bases?
 Arrhenius Acids: substance that dissolves in water to produce H ions +
-
 Ar+henius Bases: substance that dissolves in water to produce OH ions
 H ion does not exist in water because it reacts immediately with H O mol2cule to
give a hydronium ion (H O 3 +
 Arrhenius reactions tied to reactions that take place in water
 Curved arrows show change in position of electron pairs during the reaction
o Change in position of electron pair from an atom will form a new bond
o Change in position of electron pair from a bond will break that bond
What Are Bronstead-Lowry Acids & Bases?
 Bronstead-Lowry Acid: proton donor
 Bronstead-Lowry Base: proton acceptor
 Conjugate Base: species formed when an acid donates a proton
 Conjugate Acid: species formed when a base accepts a proton
 Neutralization of an acid by a base is a proton-transfer reaction in which the acid
is transformed into its conjugate base, and the base is transformed into its
conjugate acid
 Stronger the acid, the weaker is its conjugate base
How Do We Measure The Strength Of An Acid / Base?
 Strong Acid: acid that is completely ionized in aqueous solution
 Strong Base: base that is completely ionized in aqueous solution
 No tendency for the reverse reaction to occur
 Weak Acid: acid that only partially ionizes in aqueous solution
 Weak Base: base that only partially ionizes in aqueous solution
 Most organic acids and bases are weak
- +
 HA + H O2 A + H O 3
 Strength of a weak acid is expressed by its ionization constant K a
 K a K [eqO]2= [H O ]3A ] / [HA]
 pK a -log K10 a
 Larger the value of pK ,athe weaker is the acid (smaller K ) a
 Acids with K values less than 1 are considered weak acids
How Do We Determine The Position Of Equilibrium In An Acid-Base Reaction?
 In an acid-base reaction, the position of equilibrium always favours reaction of
the stronger acid and stronger base to form the weaker acid and weaker base
 At equilibrium, major species present are the weaker acid and weaker base
What Are The Relationships Between Acidity & Molecular Structure?  Most important factor in determining the relative acidities of organic acids is the
relative stability of the anion